A highly monochromatic visible light source capable of a W-class high output is considered a requisite for realizing a large-size display, a high-brightness display and the like. Among the three primary colors of red, green and blue, as a red light source, a red high-power semiconductor laser used in DVD recorders and the like is usable as a highly productive compact light source. However, green and blue light sources are difficult to realize using a semiconductor laser and the like, resulting in demands for highly productive compact light sources. In particular, green light sources are difficult to realize due to the lack of appropriate material constructible as a semiconductor laser for generating green output light.
As such a light source, a wavelength converting apparatus that combines a fiber laser and a wavelength converting element is realized as a low-power visible light source. Blue and green compact light sources which use a semiconductor laser as an excitation light source for exciting the fiber laser and a nonlinear optical crystal as the wavelength converting element are well known.
However, several challenges must be overcome in order to obtain green and blue W-class high-power output lights from such a wavelength converting apparatus. A schematic configuration of a conventional wavelength converting apparatus is shown in FIG. 31. Using this configuration, a case will now be described in which, for example, a green output light is obtained. The wavelength converting apparatus shown in FIG. 31 comprises a fiber laser 20 which outputs a fundamental wave and a wavelength converting element 27 which converts the fundamental wave into green laser light.
Furthermore, basic laser operations of the fiber laser 20 will be described. In FIG. 31, first, excitation light from an excitation laser light source 21 is incident from one end of a fiber. After the incident excitation light is absorbed by a laser active substance contained in a Yb fiber 14, a seed light of the fundamental wave is generated inside the fiber 14. The seed light of the fundamental wave is repeatedly reflected and travels back and forth inside a laser resonator which includes a fiber grating 22 and a fiber grating 25 as a pair of reflecting mirrors. At the same time, the seed light is amplified by a gain attributable to the laser active substance contained in the fiber 14, and reaches laser oscillation with its light intensity increased and wavelength selected. The laser light source 21 is current-driven by an excitation laser current source 31.
Next, basic operations of the wavelength converting apparatus shown in FIG. 31 will be described. As described above, a fundamental wave is outputted from the fiber laser 20 and enters the wavelength converting element 27 via a lens 26. The fundamental wave from the fiber laser 20 is converted into a harmonic wave by a non-linear optical effect of the wavelength converting element 27. While a portion of the converted harmonic wave is reflected by a beam splitter 28, the transmitted harmonic wave becomes a green laser light that is the output light of the wavelength converting apparatus.
The harmonic wave partially reflected by the beam splitter 28 is received by a light-receiving element 29 for monitoring output light of the wavelength converting apparatus, and subsequently converted into an electric signal to be used. An output controller 30 adjusts a driving current of the laser light source 21 using the excitation laser current source 31 so that the intensity of the converted signal enables a desired output to be obtained by the wavelength converting apparatus. Accordingly, the intensity of the excitation light from the laser light source 21 is adjusted, the output intensity of the fundamental wave of the fiber laser 20 is adjusted, and as a result, the intensity of the output of the wavelength converting apparatus is adjusted. Consequently, a so-called automatic power control (hereinafter abbreviated as “APC”) operates stably in which the intensity of the output of the wavelength converting apparatus is kept constant.
As described above, since methods such as monitoring the output from a laser light source and feeding back the same to a current value that drives the laser in order to achieve a constant light output from the laser, keeping the temperature of a laser-holding portion constant, and the like are important techniques in the field of optical recording, various methods have been conventionally proposed. For example, in Patent Document 1, a method is proposed for predicting a temperature rise of a semiconductor chip portion from a current value applied to a laser diode to regulate temperature. Various other methods have been proposed, including a method proposed in Patent Document 2 in which an upper limit is set to a current value applied when controlling light intensity using feedback control to protect a laser diode.
In addition, a method is proposed in Patent Document 3 for determining a current amount to be applied when using an air-cooled laser diode by monitoring the temperature of the laser diode instead of performing current feedback using a photodiode. Patent Document 4 proposes a method for preventing the destruction of a laser diode when commencing temperature regulation concurrently with the start of an operation of the laser diode by reducing an initial driving current in accordance with a detected temperature of the laser diode. Patent Document 5 proposes a method for determining a current amount to be applied by monitoring the temperature of a laser diode using a temperature detector to be used when performing temperature regulation of a laser. Furthermore, a configuration such as those described in Patent Documents 6 to 8 is proposed in regards to an output stabilizing method in a case of combining a laser diode with a wavelength converting element. Various methods other than those described in the aforementioned patent documents have been proposed in regards to temperature regulation of a laser diode.
However, with the conventional wavelength converting apparatuses described above, it is difficult to obtain green light in a stable manner amidst fluctuations in ambient temperature and, in particular, when the aforementioned conventional wavelength converting apparatuses are placed inside a commercially-available device such as a backlight of a projection display or a liquid crystal display, there is a problem in that a gradual rise in the temperature inside a chassis causes a decline in green output. On the other hand, a method for controlling the temperature of a wavelength converting element to a constant value, a method for feeding back an output value to LD current, and the like have been proposed in consideration of such a problem. However, the method for controlling the temperature of a wavelength converting element requires that control of the temperature of the wavelength converting element be performed in 0.01° C. increments and is not a viable option due to cost and the like insofar as its use in commercially-available devices, and the method for feeding back an output value to LD current only amounted to a compensation of around 0.3° C. and was therefore not an effective improvement. In particular, when using a fiber laser light source with a wavelength selected by a fiber grating, since the wavelength characteristics of a wavelength converting element vary according to temperature and the wavelength characteristics of the fiber grating also vary according to temperature, output stabilization cannot be achieved even when performing conventional temperature constant-value control.
Furthermore, for the purpose of improving conversion efficiency from a fundamental wave laser light source prior to wavelength conversion to a green light output that is a second harmonic wave, a wavelength converting apparatus provided with two wavelength converting mechanisms is proposed in which a fundamental wave not converted by a first wavelength converting mechanism (first stage) is once again wavelength-converted by a second wavelength converting mechanism (second stage) (for convenience sake, such a configuration shall be referred to, for convenience, as a “two-state configuration”). This two-stage configuration is characterized in that a second harmonic wave output of the second stage fluctuates dependent on a second harmonic wave output of the first stage. As such, it is difficult to control a summed value of outputs of the first and second stages using a conventional output stabilization method. In particular, with a two-stage configuration, since the output fluctuations in a harmonic wave output of the first stage and a harmonic wave output of the second stage move opposite to each other in most cases, control using current value feedback to a laser diode or normal element temperature regulation is extremely difficult.
Patent Document 1: Japanese Patent Laid-Open No. H01-098282
Patent Document 2: Japanese Patent Laid-Open No. H02-253969
Patent Document 3: Japanese Patent Laid-Open No. 2004-103954
Patent Document 4: Japanese Patent Laid-Open No. 2004-356579
Patent Document 5: Japanese Patent Laid-Open No. 2005-311133
Patent Document 6: Japanese Patent No. 3329446
Patent Document 7: Japanese Patent No. 3334787
Patent Document 8: Japanese Patent No. 3526282